Phosphorus (abbreviated as P) is a highly toxic element, which, when it occurs in the form of a phosphate salt, becomes a nutrient essential for human health. Phosphates are found in practically every type of food and, consequently, are plentiful in the typical diet. Inorganic phosphates (abbreviated as Pi) are absorbed from food as electrically charged salt anions. The organic phosphates (Po) that exist in cells and extracellular compartments of foods are primarily converted to P i through digestive processes prior to absorption. A few organic phosphates are apparently absorbed as part of small fat-soluble organic molecules, such as the phosphates of phospholipids. The concentration of P o molecules, however, is not under homeostatic control, in contrast to the concentration of Pi, which is regulated along with calcium (Ca) in blood and extracellular fluids.

The close association between calcium and P i in the extracellular body fluids and in bone tissue requires joint consideration of dietary calcium and dietary phosphates for an understanding of their physiological linkages and the important relationship between low calcium intakes and high phosphate intakes. This relationship potentially contributes to altered calcium homeostasis and the loss of bone mass.

Several aspects of P i are reviewed here in an attempt to place these essential chemical anions in the perspective of their utilization in human health and disease. The physiological functions of P are reviewed first, and a number of general references have also been included in the bibliography for the interested reader.